This proposal is a study of the genetic control of development in a simple higher organism. The proposal is based on the premise that an understanding of development at the cytological and biochemical levels will be facilitated by the use of developmental mutants. The organism of choice is a small worm which can be propagated easily in the laboratory and from which mutants can be derived. Initially, three methods for selecting temperature sensitive mutants are proposed. These methods are designed to yield mutants that are blocked in early stages of embryogenesis or mutants with altered developmental phenotype, or both of these conditions. We propose to analyze the timing of the temperature sensitive lesion with respect to development, to investigate cytologically the pattern of cell cleavages, and to study the underlying biochemical reactions of the neuromuscular system in the developing wild type and mutant embryos.